1. Field of the Invention
The present invention relates to a method and apparatus for ion attachment mass spectrometry, and more particularly relates to a method and apparatus for ion attachment mass spectrometry suitable for measuring the ingredients and concentration of, for example, a low concentration detected gas without causing dissociation.
2. Description of the Related Art
An ion attachment mass spectrometry apparatus has the advantage of enabling quantitative analysis of a detected gas without causing dissociation. In the past, some ion attachment mass spectrometry apparatuses have been reported in Hodge, Analytical Chemistry, vol. 48, no. 6, p. 825 (1976); Bombick, Analytical Chemistry, vol. 56, no. 3, p. 396 (1984); and Fujii et al., Analytical Chemistry, vol. 1, no. 9, p. 1026 (1989), Chemical Physics Letters, vol. 191, no. 1.2, p. 162 (1992), and Japanese Unexamined Patent Publication (Kokai) No. 6-11485.
FIG. 9 schematically shows an example of the basic configuration of a conventional ion attachment mass spectrometry apparatus. As shown in FIG. 9, an apparatus vessel 10 is formed by an ionization chamber 11, a differential evacuation chamber 12, and a mass spectrometry chamber 13 connected in the cascade structure. The differential evacuation chamber 12 and mass spectrometry chamber 13 are respectively provided with a differential evacuation chamber vacuum pump 14 and mass spectrometry chamber vacuum pump 15. A first aperture 16 is arranged between the ionization chamber 11 and differential evacuation chamber 12, while a second aperture 17 is arranged between the differential evacuation chamber 12 and mass spectrometry chamber 13. The ionization chamber 11 is provided with an emission mechanism 20 comprised of an ion emitter 18 and a repeller 19. Further, the emission mechanism 20 is provided with an emission mechanism control power source 21. The ionization chamber 11 has a sample gas introduction mechanism 22 and a third component gas introduction mechanism 23, which are both connected to it. A sample gas and third component gas are introduced from these introduction mechanisms 22 and 23, respectively. In the sample gas introduction mechanism 22, reference numeral 24 designates a sample gas cylinder and 25 a valve. In the third component gas introduction mechanism 23, reference numeral 26 designates a third component gas cylinder and 27 a valve. The differential evacuation chamber 12 has a focusing lens 28 arranged in it. Reference numeral 29 designates a path of metal ions and a gas which should be detected and to which the metal ions are attached. The mass spectrometry chamber 13 is provided with a Q-pole type mass spectrometer 30. At the exit side of the Q-pole type mass spectrometer 30 is provided with an ion trap 31. The output section of the ion trap 31 is connected to a data processor 32.
The ion emitter 18 of the emission mechanism 20 is made of a material including an oxide of an alkali metal. The material comprising the ion emitter 18 is for example a mixture of an Li oxide, Si oxide, and Al oxide. When the ion emitter 18 placed on the axis of the apparatus vessel 10 is heated to about 600° C. by electric power supplied from the emission mechanism control power source 21, Li+ or other positively charged metal ions are emitted into the space. These metal ions move toward an opening 16a of the first aperture 16 due to the electric field and flow of gas. During this moving period of time, the metal ions attach to the gas to be detected, which is introduced into the ionization chamber 11 as a sample gas by the sample gas introduction mechanism 22. In this way, the gas ionized by the attachment of metal ions is produced. For example, H2O becomes H2OLi+ of a mass number of the 18 amu (atomic mass units) of H2O plus the 7 amu of Li, that is, 25 amu. The positively charged ionized gas to be detected moves as it is and passes through the opening 16a. The above-mentioned path 29 shows the path of the metal ions and the gas with the metal ions attached.
When the metal ions attach to the molecules of the gas to be detected, they extremely gently attach to the locations of the charges biased on the gas molecules and almost no dissociation occurs. The smaller the bond energy, however, the easier the re-detachment of the Li+. To prevent this, it is necessary to raise the pressure in the ionization chamber 11 to the value included in the range of 10–1000 Pa (usually 100 Pa) by the third component gas introduction mechanism 23 and use collision with the gas in order to absorb the excess energy. The third component gas which also can be defined as excess-energy absorbing gas, is one of various inert gases, such as N2, which it relatively hard for the metal ions to attach to. The gas with the metal ions stably attached thereto passes through the differential evacuation chamber 12 where the focusing lens 28 is arranged. The gas subsequently enters the mass spectrometry chamber 13 where it is separated from the other gases so as to be detected in every mass through the Q-pole mass spectrometer 30.
When detecting a low concentration gas to be detected by use of the conventional ion attachment mass spectrometry apparatus shown in FIG. 9, an interference peak is sometimes caused and measurement of the signal about the detected gas becomes impossible due to concealment by the interference peak. There are four reasons for the appearance of the interference peak at this time, that is, (1) a macromer produced by third component gases, (2) a macromer produced by a third component gas and a high concentration ingredient, (3) surface ionization ions, (4) an isotope of metal ions.
Here, the “macromer” signifies a substance of two (dimer) or more gas molecules bonded together. For example, water is normally H2O, but becomes (H2O)2 as a dimer. Nitrogen is normally N2, but becomes (N2)2 as a dimer. In an ion attachment mass spectrometry method, there is the problem that even when there is actually no macromer, a slight amount of a macromer is finally produced in the process of ionization. For example, in the case of water, not only the usual H2OLi+, but also the dimer (H2O)2Li+ appears, while in the case of nitrogen, not only the usual N2Li+, but also the dimer (N2)2Li+ appears.
Further, the “surface ionization ions” signify ions produced by removing some atoms from the molecule of the gas when the gas comes in contact with a heated surface. In the ion attachment mass spectrometry method, there is the problem that after all the surface ionization ions are produced at the surface of the heated ion emitter 18 depending on the gas. For example, in the case of dimethylphthalate (C10H10O4=194 amu), ions of 163 amu being less than the inherent mass number by exactly OCH3 (31 amu) appear.
Further, the “isotope” signifies the same element with a different mass number. In the case of Li, most of them have the mass number of 7 amu, but an isotope with a mass number of 6 amu is also present in an amount of about 7.5%.